Java - Bug in Shop System

so I have a little inventory/shop interaction which is I can't find a solution for.

The player is supposed to be able to buy and sell items with a predefined buy/sell value in my Item class constructor. With the help of a scanner I'm able to sell without any bugs. However if I buy any quantity, the entire quantity is bought somehow. The item quantity is set to 0 after I buy any quantity once.

	private void buyItem() {
System.out.println("Which item would you like to buy?");
showStock();
@SuppressWarnings("resource")
Scanner sc = new Scanner(System.in);
String itemName = "" + sc.next();
for (int i = 0; i < store.length; i++) {
if (store[i] != null && store[i].getName().equals(itemName)) {
// Checks if the given item name is available in the store
System.out.println("We have " + store[i].getQuantity() + " " + store[i].getName() + "s in our stock. " + store[i].getBuyPrice() + " each");
System.out.println("How many would you like to buy?");
int buyQuantity = sc.nextInt();
// Checks if there is enough in the stock
if (buyQuantity > store[i].getQuantity()) {
System.out.println("Not enough " + store[i].getName() + " available in the stock.");
break;
} else {
// Checks if Player can afford it
if (Player.getInstance().getCoins() >= store[i].getBuyPrice()) {
// Update player inventory
Player.getInstance().removeCoins(buyQuantity * store[i].getBuyPrice());
Item item = store[i];
item.setQuantity(buyQuantity);
Player.getInstance().getInv().addItem(item);
// Buy transaction
store[i].setQuantity(store[i].getQuantity() - buyQuantity);
System.out.println("You have purchased " + buyQuantity + "x " + itemName);
System.out.println("Coins: " + Player.getInstance().getCoins());
// Checks if the entire amount has been bought
if (buyQuantity == store[i].getQuantity()) {
store[i] = null;
System.out.println("That's all " + itemName + " the store had");
}
break;
} else {
System.out.println("You don't have enough coins to buy " + buyQuantity + " " + itemName + "s");
break;
}
}
}
}
}

And this is my console output from an example:

Which item would you like to buy?
Slot 0: Weapon
Slot 1: Cake
Slot 2: Arrow
Arrow
We have 100 Arrows in our stock. 2 each
How many would you like to buy?
10
You have purchased 10x Arrow
Coins: 980
That's all Arrow the store had

I've been trying and thinking for quite some time, but I can't figure out my bug. Do you have any suggestions? I tried debugging and everything was "working" the values were correct. But when I entered the quantity I would like to buy, my int output didn't affect anything. Would highly appreciate any tip!

The line

Item item = store[i];

does not create a copy, but references the same object as is stored in the array store. The following

item.setQuantity(buyQuantity); 

sets the quantity of the item in the store to buyQuantity and the following

store[i].setQuantity(store[i].getQuantity() - buyQuantity);

effectively reduces the count of the item to 0 (both in the store and in the player inventory).

As for the fix, one could use clone() but this is regarded as broken.

I would suggest a semantically different approach. Each item does not represent a single item, but a bundle of items. Thus, a specific amount can be added or removed to/from the bundle. For this to be feasible, I would make the common superclass Item (which I assume exists) generic in T extends Item<T>, abstract and give it a public abstract T take(int amount); method:

public abstract class Item<T extends Item<T>> {
...
public abstract T take(int amount);
...
}

Then, I would change all subclasses to extend this superclass, e.g.

public class Arrow extends Item<Arrow> {
public Arrow take(int amount) {
...
}
}

Finally, I would rename the classes to reflect clearly that each Item is a bundle of items. E.g., Item can be renamed to ItemBundle and Arrow can be renamed to ArrowBundle or Arrows.

You are explicitly changing the quantity of the purchased item to the amount requested by the player here:

Item item = store[i];
item.setQuantity(buyQuantity);

If you want to add x amount of Item the the player's inventory you need to create a new instance of that Item on which you set the buyQuantity to whatever amount was purchased. That way you keep the integrity of the store's stock.